Stent

Abstract: A stent (20) is made from a wire by winding the wire on a mandrel (22), heating to form a coiled spring, and reversing the winding direction of the coiled spring to form the reversed coiled spring stent (20). The stent so formed may be reheated over a special mandrel so as to partly relax the outer portion of some or all of the stent coils. The stent may be made up of two or more sections, with adjoining sections wound in opposite senses. Such a stent may be deployed by winding the stent onto a catheter, immobilizing the two ends of the wire and one or more intermediate points, bringing the stent to the location where it is to be deployed, and releasing first the intermediate point or points and then the end points. The release of the wire may be accomplished by heating the thread immobilizing the wire so that the thread breaks and releases the wire.

Description: The present invention relates to stents and, more particularly, to methods of fabricating and deploying stents. The term "stent" has come into widespread use to denote any of a large variety of spring-like support structures, in the form of a tube which is open at both ends, which can be implanted inside a blood vessel or other tubular body conduit, to help keep the vessel or conduit open. Stents may be used following balloon angioplasty to prevent restenosis and may, more generally, be used in repairing any of a number of tubular body conduits, such as those in the vascular, biliary, genitourinary, gastrointestinal and respiratory systems, among others, which have narrowed, weakened, distorted, distended or otherwise deformed, typically as a result of any of a number of pathological conditions. An effective stent must possess a number of important and very specific characteristics. Specifically, the stent should be chemically and biologically inert to its surroundings and should not react with, or otherwise stimulate, the living tissues around it. The stent must further be such that it will stay in the correct position and continue to support the tubular body conduit into which it is implanted over extended periods of time. Further, the stent must have the ability to return to its prescribed in-place diameter after the stent diameter has been significantly reduced prior to its insertion, usually tightly wrapped on a catheter, into the tubular body conduit. These requirements limit the suitable metal stent materials to just a few metals and alloys. To date, it has been found that various alloys of nickel and titanium (hereinafter "nitinol"), with or without certain coatings, have the desired properties and are considered suitable for use in stent applications. Specifically, nitinols, with or without special coatings, have been found to be chemically and biologically inert and to inhibit thrombus formation. Nitinols are, under certain conditions, also superelastic which allows them to withstand extensive deformation and still resume their original shape. Furthermore, nitinols possess shape memory, i.e., the metal "remembers" a specific shape fixed during a particular heat treatment and can resort to that shape under proper conditions. Shape-memory alloys can be formed into a predetermined shape at a suitable heat treatment temperature. At temperatures below the transition temperature range ("TTR") certain nitinol alloys are in their martensite phase wherein they are highly ductile and may be plastically deformed into any of a number of other shapes. The alloy returns to its austenite phase, returning to its original predetermined shape upon reheating to a temperature above the transition temperature range. The transition temperature varies with each specific combination ratio of the components in the alloy. The superelasticity of nitinols and their shape memory characteristics makes it possible to fabricate a stent having the desired shape and dimensions. Once formed, the stent can be temporarily deformed into a much narrower shape for insertion into the body. Once in place, the stent can be made to resume its desired shape and dimensions. Certain alloys of nickel and titanium can be made which are plastic at temperatures below about 30°C and are elastic at body temperatures, above 35°C. Such alloys are widely used for the production of stents for medical use since these nitinols are able to resume their desired shape at normal body temperature without the need to artificially heat the stent. While such stents have been proven effective, they continue to suffer from a number of disadvantages. First, there is, in certain cases, a tendency for tissue to grow in the gaps between adjoining loops of the stent. Over time, such growth could lead to the constriction, or even the complete closure, of the tubular body conduit in which the stent was introduced in order to keep open. A continuous, gap-free, tube structure with no gaps would eliminate such undesired tissue growth. However, a rigid tube would lack the highly desirable flexibility which a coiled spring configuration offers. Another disadvantage is that the techniques for locating stents in a body conduit are such that the stents are often installed at a location which is not precisely the intended optimal location. There is thus a widely recognized need for, and it would be highly advantageous to have, a stent which would be suitably flexible but which would significantly reduce, or even eliminate, the possibility of undesired tissue growth between the coils of the stent. There is further a widely recognized need for, and it would also be highly advantageous to have, a technique for installing stents which would allow the stent to be located at precisely the desired location, either by controlling the stent design or by devising adequate methods for its accurate release. Furthermore, in those cases where the "shape memory" characteristic is used and the stent is to be heated in its final location in the body to cause it to resume its memorized shape, it is desired and advantageous to have a way of heating the stent which significantly reduces, or even eliminates, the chance of damaging surrounding tissue through heating which is conducted for too long and/or at temperatures which are too high. According to the present invention there is provided a method of fabricating a stent from a wire of a shape memory alloy, comprising: (a) winding the wire on a first mandrel; (b) heating the wound wire to form a coiled spring; and (c) after the coiled spring has cooled sufficiently, reversing the winding direction of the coiled spring to form the stent. Thus a stent is provided comprising a tightly coiled spring having a multiplicity of loops, wherein said loops are coiled in a stressed direction reversed from that of an original, relaxed direction of winding such that said loops press together tightly. Methods including steps (a) and (b) and corresponding stents are generally known in the art, see for example document WO-A-9116005. The present invention successfully addresses the shortcomings of the presently known stents and their methods of production by providing a stent which is suitably flexible but which is sufficiently tight so as to eliminate the gaps between adjoining windings of the stent, thereby significantly reducing or even eliminating the possibility of undesirable growth of tissue between winding of the stent. The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: FIG. 1 is a perspective view of a single winding of a prior art stent; FIG. 2 is a perspective view of a single winding of stent according to the present invention which was obtained by reversing the winding of a stent such as that in Figure 1; and FIG. 3 is a side cross sectional view of a stent undergoing reheating according to the present invention, on a mandrel having two sections, each with a different heat sink capacity. Referring now to the drawing, Figure 1 illustrates a single winding of a conventional stent. In many applications, it is important to precisely control the flexibility of the stent as well as the interloop spacing and tightness. A number of factors must be considered in selecting the proper flexibility and interloop spacing. First, the stent must be sufficiently flexible to follow the natural shape and dimensions of the body conduit into which it is installed without undue stress. The stent must also be sufficiently flexible to adequately follow the various movements of the conduit. These requirements tend to indicate that a coiled, or spring-like, structure be used. However, the stent must not be too loose since this may erode its body conduit support function and since when the stent loosens significant interloop gaps are formed which tend to encourage the growth of surrounding tissue into the separations. Such ingrowth may have serious adverse consequences. A stent is typically made by first tightly winding a wire of a suitable material, such as nitinol, on a mandrel. The assembly is then heated to a suitable temperature so as to impart to the stent its desired shape. However, during the heating process, the mandrel is also heated, which brings about its expansion and leads to the formation of a stent with loops which are somewhat separated from one another. Such separations are undesirable in certain applications. These interloop gaps can be eliminated and the stent can be stiffened somewhat by reversing the winding direction of the stent after it has cooled sufficiently. Shown in Figure 2 is the single stent winding of Figure 1 after it has been reversed. Thus, what, prior to reversal (Figure 1), was the left end of the loop, 10 , is, after reversal (Figure 2), the right end of the loop, 10 , while what, prior to reversal (Figure 1), was the right end of the loop, 12 , is, after reversal (Figure 2), the left end of the loop, 12 . As will be appreciated, the reversal puts each loop in elastic deformation and thereby causes adjoining loops to press together and eliminates interloop gaps. Under certain conditions a stent made by reversing the winding direction as described above may be overly rigid for a specific application. In such a case, the rigidity of the stent may be reduced to any desired level by following a reheating procedure described below. The reversed stent 20 , whose rigidity is to be reduced, is mounted onto a mandrel 22 (Figure 3) which may or may not be the mandrel previously used to give the stent its original shape. Stent 20 and mandrel 22 are reheated at a suitable temperature above the transition point but the reheating is allowed to continue only long enough to allow the outside portion of the stent (indicated in Figure 3 as the unhatched portion) to approach the reheating temperature and therefore to relax, while the portion of the stent near the relatively cool mandrel (indicated in Figure 3 by hatch marks) stays at significantly lower temperatures, does not relax, and continues to have its original rigidity. In this way the reheated stent, upon cooling, displays a flexibility which is intermediate between those of the unreheated stent and a stent which is completely relaxed, but without opening up gaps between the stent loops. The duration of the reheating must be carefully controlled to achieve the proper degree of relaxation. The reheating time will be influenced to a large degree by the heat properties of the mandrel. A mandrel which has high heat sink capacity, such as the left-hand portion of the mandrel of Figure 3, can absorb considerable heat and keep the stent at low temperatures for a relatively long time. By contrast, a mandrel which has low heat sink capacity, such as the right-hand portion of the mandrel of Figure 3, can absorb very little heat and will not keep the stent at low temperatures but rather will allow that portion of the stent overlying it to quickly reach the overall heating temperature and undergo complete relaxation. Advantage may be taken of this property to reheat different portions of a stent to different extents so as to achieve a final product which has a certain rigidity in one or more sections and is relaxed and features significant interloop gaps in other sections. Typically, it may be useful to have significant interloop gaps between the windings near each end of the stent to facilitate the suturing of the stent in place. It will be appreciated that a stent having regions of differing relaxation characteristics can also be achieved by heating the different segments to different temperatures and times, such as by use of a segmented heater or furnace.

Claims: A method of fabricating a stent from a wire of a shape memory alloy, comprising (a) winding the wire on a first mandrel; (b) heating the wound wire to form a coiled spring; and (c) after said coiled spring has cooled sufficiently, reversing the winding direction of said coiled spring to form the stent. A method according to claim 1, further comprising: (d) placing said stent on a second mandrel; and (e) reheating said reversed coiled spring stent for a time sufficiently short as to partially relax said stent. A method according to claim 2, wherein reheating comprises reheating to a temperature above a transition temperature for the material of the stent. A method according to claim 2 or 3, wherein reheating comprises reheating such that the stent has regions of differing relaxation characteristics. A method according to claim 4, wherein said second mandrel includes at least two regions of different heat sink capacities, the method further comprising relaxing the portion of said stent overlying said at least two regions to different extents during said reheating by virtue of said different heats fink capacities. A method according to claim 4, wherein reheating comprises reheating different regions of the stent to different temperatures and times. A method according to claim 6, wherein reheating comprises reheating using a segmented heater. A method according to claim 4, wherein reheating includes reheating said reversed coil spring such than significant interloop gaps exist between windings of the stent near each end thereof. A method according to claim 2 or 3, wherein reheating comprises reheating long enough to allow the outside portion of the stent to approach the reheating temperature so as to relax the outside portion, while keeping the portion of the stent near the mandrel at lower temperatures so as to keep the portion of the stent near the mandrel at its original rigidity. A stent, formed of a shape memory alloy, comprising a tightly coiled spring having a multiplicity of loops, wherein said loops are arranged in a coil in a stressed direction reversed from that of a relaxed direction of winding such that said loops press together tightly. A stent according to claim 10, wherein said coiled spring is partially relaxed. A stent according to claim 10, wherein the stent has regions of differing relaxation characteristics. A stent according to claim 10, wherein significant interloop gaps exist between windings of the stent near each end thereof. A stent according to claim 10 wherein the outside portion of the stent is more relaxed than the inside portion of the stent.

Inventors: PINCHASIK GREGORY, PINCHASIK, GREGORY
Applicants: MEDINOL LTD, MEDINOL LTD.
Application year: 1994
Main classification: A61F2
IPC codes: B21F3, A61F2